1. Field of the Invention
The present invention relates to a thread tension device, and more particularly to a thread tension device for a sewing machine, particularly an overlock sewing machine or an overedge sewing machine.
2. Description of the Prior Art
A typical thread tension device for an overedge sewing machine is disclosed in U.S. Pat. No. 4,803,936 to Mikuni et al. and comprises a number of tension disc assemblies disposed on a shaft for clamping the threads and each including a first thread tension setting device for setting the spring force against the pair of disc of the tension disc assemblies, and each including a second thread tension setting device for adjusting the tension of the first thread tension setting device, and each further including a thread tension releasing device for releasing the spring forces to the tension disc assemblies. The thread tension of one or more of the tension disc assemblies may be released simultaneously.
Another typical thread tension device for an overlock sewing machine is disclosed in U.S. Pat. No. 5,265,548 to Satoma and also comprises a thread tension unit for clamping a thread to apply tension, a first thread tension setting device for setting the thread tension unit to apply a preset thread tension, a second thread tension setting device for adjusting the tension applied by the thread tension unit to an optional tension, and a thread tension nullifying device for nullifying the setting made by the second thread tension setting means. The thread tension nullifying devices each includes a pair of rocking bodies directly engaged with a cam body such that each of the thread tension units requires a whole set of rocking bodies and cam body.
In both of the prior arts, a whole set of tension releasing device or nullifying device is required to be provided for each pair of the clamping discs such that a number sets of identical tension releasing or tension nullifying devices are required to be engaged in the upper portion of the sewing machine. However, the upper portion of the sewing machine normally includes a compact or tiny space that may not be easily installed with so many elements and members.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the conventional tension devices for sewing machines.